When working with circles there are several circumastances which you need to know:

	The angle at the centre is twice the angle at the circumference for angles which stand on the same arc.
	Angles at the circumference which are in the same segment are all equal.
	An angle at the circumference which stands on the diameter of the circle is always a right-angle.
	Opposite angles of a cyclic quadrilateral (all verticies on the circumference) add up to 180° ( d+f = e+g = 180 ).
	A tangent meets the radius of a circle at 90°.